Shows charge symbol but it is not charging, any ideas?

I am a smartphone and tablet repair technician so I have done close to a hundred iphone 4 and 4s. This 4s need the charge port flex replace because it wasn't charging. After the replacement was done, I plugged in the charge cord and the phone got stuck in a boot loop. After troubleshooting it for a few minutes I decided to change the charge port again. Installed a new one and this time when I plugged the cord in the phone booted up fine. I let it sit on the charge for a few minutes while I did other repairs. When I came back to the phone I notice that the battery percentage did not go up at all, so I place a new battery in and repeat the charge process. Wher I return to it again, same problem, battery percentage did not increase it actually lowered a percent. The charge symbol is displayed but not charging. Since that I have tried 2 new batteries, 3 different cords and chargers, home+wake/sleep restart, and put it into restore mode (cancelled that before actually restoring) so I am stuck.